By that logic, all countries in the world besides China and the USA could stop reducing emissions because they only cause sub 10% shares of the total.

No, because China and the USA are both affected by the emissions regardless of which one of them are responsible for them. In that case, the one we're actually faced with, it makes more sense to tackle the emissions that are highest first and that have the lowest barriers. You pick the problems with the largest return on investment in time and resources. Airplanes are not that. Banning commercial flights for people is a fantasy and banning private jets, although something I agree with for other reasons, is not enough to make a dent.

There isn't a whole lot "outside that 5%".

Yes, there is. Cars, on average, have not lowered their emissions at anywhere near the same rate as airplanes have over the last 20 years and that's including new electric cars. Until electric cars overtake gas-powered vehicles, which is currently projected to happen in 2031, there is enough within this sector alone that is more than 5% of the problem and that doesn't require an absolute fantasy for a solution. And that doesn't even touch manufacturing and industrial emissions which account for an even bigger slice because of the energy they use.

You're right... it all has to become 0 anyways but we don't have unlimited time or unlimited resources. Efforts need to be prioritized to put the ways that are realistic and meaningful at the top and unrealistic ways that solve 5% of the problem at the bottom.